I am building a neat headset VOX system using Arduino (maybe post the description here when it is completed). Anyway, the condenser mic is powered from USB supply via 10k resistor and connected to LM358 preamp via 1uF capacitor. The problem arises when the mic input picks up also the ripple from the USB switching regulator. Measured with DSO, the USB charger outputs nice 5 V +- 20 mV ripple with lot of harmonics.
Replacing the USB wall-socket charger with an external 1600 mAh USB powerpack is not going to help either - there is another DC-DC-switching regulator inside the powerpack, and the problem persists...
Is there any way to clip or filter the input voltage to get rid of the harmonics ? A 4.7 V zener diode comes immediately to the mind but does it help ?